GRIN lens structure in micro-LED devices

ABSTRACT

A GaN layer of micro-LEDs is exposed to ion implantation to amorphize one or more regions of the GaN layer. As a result, the GaN layer through which light rays propagate have non-uniform refractive indexes that modify propagation paths of some light rays. Ions are implanted in a region overlapping an active region that emits light to function as a converging GRIN (gradient-index) lens. The ion implanted regions collimate light rays that propagate along predetermined directions. As such, the light extraction from and the focus of the micro-LEDs is increased.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No.15/824,970 filed on Nov. 28, 2017.

BACKGROUND

The present disclosure generally relates to micro-LEDs (lightingemitting diodes), and specifically to exposing a GaN layer to ionimplantation to increase light extraction from and the focus of themicro-LEDs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of an example micro-LED, according toone embodiment.

FIGS. 1B and 1C are top views of example micro-LEDs, according todifferent embodiments.

FIG. 2A is a cross-sectional view of an example micro-LED, according toone embodiment.

FIGS. 2B and 2C are top views of example micro-LEDs, according todifferent embodiments.

FIG. 3A is a cross-sectional view of an example micro-LED, according toone embodiment.

FIGS. 3B and 3C are top views of example micro-LEDs, according todifferent embodiments.

FIGS. 4A through 4C illustrate an example process of manufacturing anexample micro-LED by exposing a GaN layer to ion implantation, accordingto one embodiment.

The figures depict embodiments of the present disclosure for purposes ofillustration only.

DETAILED DESCRIPTION

The light extraction efficiency of micro-LEDs may be limited by totalinternal reflection caused by the difference between the refractiveindex of crystalline GaN materials and that of the output materials(e.g., air, sapphire, silicon carbide). Conventional approachesphysically etch structures that redirect light rays generated at angleslarger than the escape cone into a shallower angle to increase the lightextraction from micro-LEDs. The physically etched structures allow therays to exit from the substrate whilst increasing the effective size ofthe micro-LEDs (mLEDs). However, an increase in the non-radiativerecombination on the etched surface may result in a reduction in theintrinsic quantum efficiency of the LED.

In addition, it is advantageous to form a tightly focused spot and havea suitable depth of field at the same time for pattern programmablelight sources. One of many approaches to achieve this is to use shapedetched pixels that “funnel” the emitted light into a well-defined beam.However, this approach has its disadvantages. First, it is challengingto determine high resolution features to be transferred into the GaNmaterials and unwanted recombination effects at the sidewalls of themLEDs. Secondly, to provide a well-defined beam, this approach uses LEDmaterials that include thick layers of compound semiconductor material,which is not applicable to readily available commercial GaN LED wafers.

Embodiments related to exposing a GaN layer of mLEDs to ion implantationthereby to amorphize one or more regions of the GaN layer. As a result,the GaN layer through which light rays propagate have non-uniformrefractive indexes that modify propagation paths of some light rays. TheGaN layer is exposed to ion implantation such that one or more regionsof the GaN layer are amorphized for performing different opticalfunctions. In one embodiment, a region that overlaps an active regionthat emits light is exposed to ion implantation. This region functionsas a converging GRIN (gradient-index) lens. The amorphous regionscollimate light rays that propagate along predetermined directionsthereby to increase the light extraction from and/or the focus of themicro-LEDs. In one implementation, exposure to oxygen ion implantationis carried out with implantation energies such that the oxygen ions aredeposited in the local converted regions. This oxygen ion implantationconverts the crystalline GaN into an amorphous-GaN:O form by disruptingthe crystal lattice. The oxygen ion implantation also serves tostabilize this amorphous form from a self-annealing effect during theimplantation exposure.

FIG. 1A is a cross-sectional view of an example micro-LED 100, accordingto one embodiment. The example micro-LED (“mLED”) 100 is made of asemiconductor material and includes a p-n junction. The example mLED 100comprises a multi-layer epitaxial thin film structure as furtherdescribed below in connection with FIGS. 4A through 4C and is depositedonto a substrate (not shown). The substrate can be made of any suitablematerial, such as sapphire, silicon carbide, gallium nitride, silicon,or any other substrate material demonstrated in the art for growth ofepitaxial structures. To facilitate discussion, a bottom of the mLED isdefined as the surface that contacts the substrate onto which the mLED100 is deposited.

From top to bottom, the example mLED 100 includes a metal contact 102that is deposited on a p-type GaN layer 104 to allow electricalstimulation of the mLED 100. The metal contact 102 may be made ofaluminum, silver, or other conductive materials. The metal contact 102may function as a reflective layer that reflects some light generated bya quantum well layer 106 towards the bottom of the mLED. The p-type GaNlayer 104 is used as a p-type contact. The p-type GaN layer 104 may bedoped with magnesium (Mg) and may have a thickness in the range of 0.05to 0.5 micron.

The p-type GaN layer 104 is on a quantum well layer 106. The quantumwell layer 106 may include a confined active region 106 a and one ormore non-active regions 106 b. The confined active region 106 a is theactive region between the two types of semiconductor materials (i.e.,the p-type GaN 104 and the n-type GaN 108) that emits light (e.g.,violet light, blue light, green light, red light) in response to apotential difference applied between the n-type GaN layer 102 and thep-type GaN layer 104. The electroluminescent wavelength of the lightemitted is determined by a width and composition of the quantum well.For example, a 2 nm-thick In(0.2)Ga(0.8)N quantum well emitsluminescence near 450 nm. In some embodiments, the quantum well layer106 is an InGaN/GaN (Indium gallium nitride/gallium nitride) quantumwell. The quantum well layer 106 can be a single-quantum well structureor a multi-quantum well structure. The confined active region 106 acorresponds to a pixel. The non-active regions 106 b do not emit lightas the current is channeled into the confined active region.

The quantum well layer 106 is deposited on the n-type GaN layer 108which is used as an n-type contact. The metal contact deposited on then-type GaN layer 108 is not shown. In the illustrated example, thequantum well layer 106 is grown on the n-type GaN layer 108. The n-typeGaN layer 108 has an interior surface 111 and an exterior surface 112opposite the interior surface 111. The exterior surface 112 contacts asubstrate (not shown) that supports the mLED 100. Multiple mLEDs canshare one p-type GaN layer 104 (or n-type GaN layer 108). That is, thep-type GaN layer 104 (or n-type GaN layer 108) can extend acrossmultiple mLEDs. In this way, the multiple mLEDs can be electricallystimulated simultaneously. For example, when mLEDs are arranged to forma display, the mLEDs share a common n-type GaN layer 108 but haveisolated p-type GaN layers. The mLEDs 100 can be independentlycontrolled by applying different voltage to their respective p-type GaNlayer.

One or more regions of the n-type GaN layer 108 are exposed to ionimplantation and thus may include implanted ions. The rest of the n-typeGaN layer 108 is not or is substantially not exposed to ionimplantation. The exposure to ion implantation amorphize the n-type GaNthat is originally crystalline and changes the refractive index of thematerial. The one or more regions of the n-type GaN layer 108 can bepartially or completely amorphous. An n-type GaN exposed to ionimplantation has a significantly lower refractive index (e.g., 2.2) thanan n-type GaN without being exposed to ion implantation. Accordingly,the refractive index is not uniform across the n-type GaN layer 108 andchanges propagation of incoming light emitted by the quantum well layer106 through the n-type GaN layer 108. As further described below, ionimplantation is performed in a variety ways to increase light extractionefficiency of the n-type GaN layer 108 and/or to increase focus ofoutgoing light exiting the n-type GaN layer 108.

During implantation, the ions can be of different energy levels suchthat the energy collision cascades damage or destroy the crystalstructure. In some cases, the implantation energy levels are configuredwith sufficient spread such that the implantation occurs within thevolume of the material (e.g., GaN). In some cases, the implantationenergy levels and the spread are configured such that the ions arestopped within a localized region of the material. The localized regionis separate and away from the volume undergoing the amorphousconversion. The ions (e.g., oxygen ions) may serve to stabilize theamorphous material.

In some embodiments, one or more regions of the n-type GaN layer 108that are exposed to ion implantation form an optical waveguide thatguides light rays to propagate along predetermined directions throughthe n-type GaN layer 108. That is, the amorphous regions 110 deflectlight rays from propagating along predetermined directions orsignificantly reduce light rays that propagate along the predetermineddirections within the n-type GaN layer 108. The regions that are exposedto ion implantation may include implantation of ions. A region that isexposed to ion implantation is also referred hereinafter as “anamorphous region” and a region that is not exposed to ion implantationis also referred hereinafter as “a crystalline region.” An amorphousregion can be partially or completely amorphous. An amorphous region ispositioned along predetermined propagation paths of incoming light raysemitted by the quantum well layer 106.

Because the amorphous region has a different refractive index than thecrystalline GaN region, the amorphous region modifies the propagationpaths of these light rays, thereby modifying the angle of incidence ofthese light rays with respect to the interior surface 111. As describedherein, an angle of incidence is the angle between a light ray incidenton a surface (e.g., the interior surface 111) and the normal directionthat is perpendicular to the surface at the point of incidence. Theamorphous regions are positioned such that reflection of light raysinternally within the n-type GaN layer 108 is reduced and a lightextraction efficiency of the n-type GaN layer 108 is increased. Therefractive index of the amorphous region is lower than that of thecrystalline region. In some embodiments, the amorphous region 110 has auniform refractive index profile. In some embodiments, the amorphousregion 110 has a graded refractive index profile. A refractive index ofthe amorphous region 110 is in the range of 2.1 to 2.45. A refractiveindex of the crystalline region is in the range of 2.38 to 2.56.

In the illustrated example, the amorphous region 110 is tube-shaped. Theamorphous region 110 partially encloses a crystalline GaN region 114through which light rays can propagate. The n-type GaN layer 108 furtherincludes a crystalline GaN region 109 that is not partially enclosed bythe amorphous region 110. The amorphous region 110 guides incoming lightrays that propagate along predetermined directions (e.g., 120, 121) topropagate through the crystalline GaN region 114 in a direction towardsthe bottom of the mLED 100. Because the refractive index of theamorphous region 110 is lower than that of the crystalline GaN region114, the amorphous region 110 deflects incoming light rays from thecrystalline GaN region 114 when the light rays are incident on a surfaceof the amorphous region 110. As such, the angle of incidence of theselight rays on the interior surface 111 of the n-type GaN layer 108 arechanged. The crystalline GaN region 114 has a refractive index in therange of 2.38 to 2.56.

In the illustrated example, the amorphous region 110 has a uniformrefractive index profile, the amorphous region 110 reflects the incominglight rays. The light rays propagate through the crystalline region 114,for example, along directions 122, 123. In other embodiments where theamorphous region 110 has a non-uniform (e.g., graded) refractive indexprofile, the light rays follow a non-linear path when propagating withinthe crystalline GaN region 114 towards the bottom of the mLED 100. Therefractive index of the amorphous region 110 varies along a firstdirection that is substantially parallel to the interior surface 111 andis substantially uniform along a second direction that is perpendicularto the interior surface 111. For example, if the amorphous region 110has a substantially parabolic refractive index profile, the light raysfollow one or more sinusoidal paths. That is, the refractive index ofthe amorphous region 110 changes as a parabolic function of the radiusdistance from the center of the mLED 100.

The amorphous region 110 may be of different depths. For example, asillustrated, the amorphous region 110 are shallower than the n-GaN layer108 in depth. During implantation, the ions do not reach the quantumwell layer 106. In other embodiments, the amorphous region 110 is asdeep as the n-GaN layer 108. In some embodiments, for example asillustrated, the interior surface of the amorphous region 110 issubstantially perpendicular to an exterior surface 107 of the quantumwell region 106. In other embodiments, the amorphous region 110 isconically-shaped.

In some embodiments, for an individual mLED, the amorphous region 110 ispositioned offset from the confined active region 106 a such that thecrystalline region 114 overlaps the confined active region 106 a. Theamorphous region 110 does not overlap the confined active region 106 a.The crystalline region 114 preferably overlaps the entire confinedactive region 106 a such that substantially all light emitted by theconfined active region 106 a is directed into the crystalline region114. The amorphous region 110 has a cross-section that is substantiallycylindrical, square, rectangular, hexagonal or other shapes. Forexample, as illustrated in FIG. 1B, a cross-section of the amorphousregion 110 is substantially square shaped. A cross-section of theamorphous region 110 is substantially cylindrical shaped in the exampleillustrated in FIG. 1C. In both examples, the centers of the quantumwell layer 106, the amorphous region 110, and the crystalline region 114are aligned. The crystalline region 114 may overlap the entire confinedactive region 106 a.

The example structure illustrated in FIG. 1A can include otheradditional layers. For example, a capping layer is between the metalcontact 102 and the p-type GaN layer 104. The capping layer is grownover the p-type GaN layer 104 and the metal contact 102 is formed on thecapping layer. The capping layer may be made of highly doped p++ GaN,indium tin oxide (ITO), or other transparent conductors. As anotherexample, an electron blocking layer (EBL) such as an aluminum galliumnitride (AlGaN) is between the quantum well layer 106 and the p-type GaNlayer 104. The EBL is grown on the quantum well layer 106 to improve theperformance of the quantum well layer 106 (e.g., the confined activeregion). These additional layers may or may not be included in theepitaxial design.

In some embodiments, an amorphous region collimates light rays thatpropagate through an n-type GaN layer. FIG. 2A is a cross-sectional viewof an example mLED that includes amorphous regions that collimates lightrays. The example mLED 200 includes a metal contact 102, a p-type GaNlayer 104, a quantum well layer 106, and an n-type GaN layer 208.Compared to the mLED 100 illustrated in FIG. 1A, the mLED 200 includes adifferent n-type GaN layer 208 than the n-type GaN layer 108. Thedescription of the metal contact 102, p-type GaN layer 104, and quantumwell layer 106 can be found with respect to FIG. 1A and is omittedherein with respect to FIG. 2A.

The n-type GaN layer 208 is used an n-type contact. The metal contactdeposited on the n-type GaN layer 208 is not shown. The n-type GaN layer208 includes an amorphous region 210 and a crystalline region 209. Theamorphous region 210 may include varying doses of ion implantation alonga plane that is parallel to the interior surface 211 of the n-type GaNlayer 208. Because the interior surface 211 of the n-type GaN layer 208is typically parallel or substantially parallel to the interior surface213 of the quantum well layer 106, the doses of ion implantation varyalong a plane that is parallel to the interior surface 213 of thequantum well layer 106. In some embodiments, the doses of ionimplantation increase radially from the center to the perimeter of theamorphous region 210. Accordingly, a refraction index decreases from thecenter to the perimeter of the amorphous region 210 radially. By havinga varying refraction index from the center to the perimeter, theamorphous region 210 collimates incoming light rays emitted by thequantum well layer 106. That is, the amorphous region 210 functions as aconverging gradient-index (GRIN) lens. By collimating incoming lightrays emitted by the quantum well layer 106, the amorphous region 210increases the light extraction efficiency of a mLED 200.

In some embodiments, the refraction index of the amorphous region 210follows a parabola (e.g., the parabola 215) such that its refractionindex varies parabolicly from the center to the perimeter of theamorphous region 210 radially. The refraction index can be determinedaccording to Equation (1):

$\begin{matrix}{{{n(r)} = {n_{1}\left( {1 - \frac{\left( {r\sqrt{A}} \right)^{2}}{2}} \right)}},} & (1)\end{matrix}$where n(r) is the index at a location that is of a distance r from thecenter of the amorphous region 210, n₁ is a refraction index of theamorphous region 210 at the center, r is the radial distance from thecenter to the perimeter of the amorphous region 210, and √{square rootover (A)} is the gradient constant describing the index variation acrossthe amorphous region 210. A refractive index of the amorphous region 210is in the range of 2.1 to 2.55.

The dimension (e.g., a width, a length) of the amorphous region 210 canbe determined based on a light extraction efficiency of the mLED 200 anda beam divergence of light emitted by the mLED 200. The dimension ischosen to maximize the light extraction efficiency and to minimize thedivergence of the light. The depth Z and the diameter D of the amorphousregion 210 can be determined according to Equations (2) and (3),respectively:

$\begin{matrix}{{Z = \frac{2\pi P}{\sqrt{A}}},} & (2)\end{matrix}$where P is the pitch of the corresponding GRIN lens that refers to thefraction of a complete sine wave cycle a ray would complete onpropagating through the lens, and √{square root over (A)} is thegradient constant describing the index variation across the amorphousregion 210. For example, a pitch P of 0.25 indicates a quarter of acomplete cycle.

$\begin{matrix}{{D = \frac{\sqrt[z]{32\left( {1 - \frac{n\; 2}{n\; 1}} \right)}}{\pi}},} & (3)\end{matrix}$where n₂ is a refraction index of the amorphous region 210 at theperimeter (i.e., r=D/2). In various embodiments, n₁ and n₂ have valuesof 2.45 and 2.2, respectively.

In one embodiment, the amorphous region 210 has a diameter of 2.6micrometers and a depth of 4 micrometers used for collimating light rayshaving wavelength of 460 nanometers. The light rays are emitted from theconfined active region 106 a of which a diameter is less than 2.6micrometers.

The amorphous region 210 is column shaped and has a cross-section thatis substantially cylindrical, square, rectangular, or other shaped. Insome embodiments, the amorphous region 210 is positioned such that itscenter is aligned with the center of the confined active region 106 aand the amorphous region 210 overlaps the confined active region 106 a,as further illustrated in FIGS. 2B and 2C. For example, as furtherillustrated in FIG. 2B, a cross-section of the amorphous region 210 issquare shaped. A cross-section of the amorphous region 210 issubstantially circular shaped in the example illustrated in FIG. 1C. Theion-implantation dose varies radially from the center to the perimeterof the amorphous region 210 as illustrated. The amorphous region 210preferably has a larger cross-section than the confined active region106 a.

Some embodiments include both types of amorphous regions as describedwith respect to FIGS. 1A through 2C for collimating light rays and forincreasing the light extraction efficiency. An example is described withrespect to FIG. 3A through 3C.

FIG. 3A is a cross-sectional view of an example mLED 300. The mLED 300includes a metal contact 102, a p-type GaN layer 104, a quantum welllayer 106, and an n-type GaN layer 308. Compared to the mLEDs 100, 200illustrated in FIGS. 1A and 2A, the mLED 300 includes a different n-typeGaN layer 308 than the n-type GaN layers 108, 208. The description ofthe metal contact 102, p-type GaN layer 104, and quantum well layer 106can be found with respect to FIG. 1A and is omitted herein with respectto FIG. 3A.

A metal contact (not shown) is deposited on the n-type GaN layer 308 isused as an n-type contact. The n-type GaN layer 308 includes a firstamorphous region 210, a second amorphous region 110, and a crystallineregion 309. The first and second amorphous regions 210, 110 aredescribed with respect to FIGS. 2 and 1A through 1C, respectively. Thefirst and second amorphous regions 210, 110 are positioned such that thefirst amorphous region 210 overlaps the confined active region 106 a andthe second amorphous region 110 does not overlap the confined activeregion 106 a. The cross-sections of the first and second amorphousregions 110, 210 are preferably the same. For example, both amorphousregions have rectangular-shaped cross sections as illustrated in FIG.3B, and both amorphous regions have circular-shaped cross sections asillustrated in FIG. 3C. As illustrated, the first amorphous region 210and the second amorphous region 110 are not contiguous, and acrystalline region 309 is positioned between the amorphous regions 210,110. However, the first amorphous region 210 and the second amorphousregions 110 can be contiguous. Other embodiments of mLEDs can be basedon other multi-layer epitaxial thin film structures that are differentfrom the examples illustrated in FIGS. 1A through 3C.

FIGS. 4A through 4C illustrate an example process of manufacturing anexample mLED by exposing a GaN layer to ion implantation, according toone embodiment. The example mLED manufactured according to the exampleprocess is based on a different structure from that of the example mLEDs100, 200, and 300 described above.

After LED epitaxial layers have been grown on the substrate 401, thesubstrate 401 is removed. The substrate 401 can be removed, for example,by laser lift-off or by grinding and wet or plasma etching. After thesubstrate 401 is removed, the n-type GaN layer 402 is exposed to ionimplantation. A mask 422 is placed onto the n-type GaN layer 402. Themask 422 is patterned such that it includes one or more regions thatallow ions to reach the n-type GaN layer 402. For example, the mask 422includes a grid of regions that allow ions to pass. The regions arepositioned the same as mLEDs positioned. That is, two consecutive masksare separated by a same distance that separates two consecutive LEDs.When placing the mask 422 onto the n-GaN layer 402, the regions arealigned to individual mLEDs.

As illustrated, the mask 422 includes regions 423, 424, and 425. Theregions 423 and 424 allow ions to reach the n-type GaN layer 402 to formone or more amorphous regions (e.g., the amorphous regions 110, 210).The region 423 has a substantially uniform thickness and the region 424has a non-uniform thickness. As illustrated, the region 424 has a slopedwall. The thickness of the region 424 decreases from its center to theperimeter and thus an ion-blockage rate decreases from its center to theperimeter. That is, the center region allows less ions to pass comparedto the perimeter region. Accordingly, across the corresponding maskedarea, the center is more crystalline than the perimeter after ionimplantation. A thickness of the region 424 may follow a parabolic curvesuch that doses of ion implantations in the corresponding masked areachange according to a parabolic curve as described with respect to FIG.2A. The region 423 allows higher doses of ions to reach the n-type GaNlayer 402 compared to the region 424. Accordingly, masked areascorresponding to the region 423 are more amorphous than the masked areascorresponding to the region 424 and thus have a lower refractive index.The masked areas corresponding to the region 424 is amorphous of avarying degree because the region 424 has a varying degree of thickness.Further, the region 425 blocks ions from reaching the n-type GaN layer402. In other embodiments, a mask includes one or more of the regions423, 424, and 425.

The mask 422 is positioned such that an individual region 424 overlaps aquantum well of an individual LED architecture and an individual region423 does not overlap the quantum well. As illustrated, the mask includesa region 425 that blocks ions between the region 424 and the region 423.The regions 424, 423 can be contiguous. In some embodiments, the maskincludes only the region 424 or only the region 423. That is, the region424 is replaced with the region 425 or the region 423 is replaced withthe region 425.

In various embodiments, the region 423 has a ring-shaped cross sectionin circular, rectangular, square, or other shapes. The region 424 has across section that is circular, rectangular, square, or other shaped.The cross section of the region 424 varies in area across a depth of themask. The cross section of the region 424 that contacts the exteriorsurface of the n-GaN layer 402 is preferably at least the same or largerthan the active light emitting region (e.g., the confined active region106 a). When placing the mask 424 onto the exterior surface of the n-GaNlayer 402, a center of the region 424 is aligned to a center of theactive region. When the mask 422 does not include any region 424, theregion 423 partially encloses an ion block region 425. A center of theregion 423 and the center(s) of the one or more region(s) that arepartially enclosed by the region 423 are aligned. For example, asillustrated, a center of the region 423 is aligned to a center of theregion 424, both of which are aligned to the center of the active regionwhen the mask 422 is placed onto the n-type GaN 402.

In the illustrated example, doses of implanted ions are regulated bycontrolling a thickness of regions of the mask 422 that allow ions topass. Other properties of the mask 422 can be regulated to control dosesof ions implanted into n-GaN layers. In some cases, the n-GaN region 402may be pre-coated with specific materials such as silicon nitride totailor the ion implantation characteristics. The ion species, dose,and/or implantation energy are selected according to a desiredrefractive index range of the amorphous form of the n-GaN region 402.The ion species may be selected to have specific properties that are ofimportance within the n-GaN region after implantation is complete suchas using oxygen ions for amorphous material stabilization. In somecases, the ions may be projected at angles to tailor specificrequirements. In some cases, the ion species, energy, projection angle,and the coating may be selected to tailor the straggle of the ionimplantation such that a conical shaped volume of the n-GaN region 402becomes amorphous.

After the mask is applied, ion implantation is performed in a directionsuch that ions are accelerated and impacted into the n-type GaN layer402 through the mask 422. As such, one or more amorphous regions areformed. The mask 422 and any pre-treatment GaN coating may besubsequently removed and the substrate is assembled back to the LEDarchitecture of which the n-type GaN layer 402 includes one or moreamorphous regions.

Other arrangements and structures can also be used. For example, invarious embodiments described herein, the lower contact is an n-typematerial and the upper contact is a p-type material. A reverse structurecan also be used such that the lower contact is a p-type material andthe upper contact is an n-type material.

The foregoing description of the embodiments has been presented for thepurpose of illustration; it is not intended to be exhaustive or to limitthe patent rights to the precise forms disclosed.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the patent rights be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thepatent rights, which is set forth in the following claims.

What is claimed is:
 1. A light emitting diode, comprising: a first layerof semiconductor material; an active region on the first layer ofsemiconductor material; and a second layer of semiconductor material onthe active region, the active region configured to emit light throughthe second layer of semiconductor material in response to a potentialdifference applied between the first layer and the second layer ofsemiconductor material; wherein the second layer of semiconductormaterial comprises a first region positioned to overlap the activeregion, wherein the first region has a refractive index profile thatchanges gradually in a radial direction from a center of the firstregion.
 2. The light emitting diode of claim 1, wherein doses ofimplanted ions in the first region of the second layer of semiconductormaterial vary radially from a center of the first region to a perimeterof the first region.
 3. The light emitting diode of claim 1, whereindoses of implanted ions in the first region of the second layer ofsemiconductor material increase from a center of the first region to aperimeter of the first region.
 4. The light emitting diode of claim 3,wherein a profile of the doses is represented as a parabolic curve thatdecreases from the center of the second layer of semiconductor materialtowards a perimeter of the second layer of semiconductor material. 5.The light emitting diode of claim 1, wherein doses of implanted ions inthe first region of the second layer of semiconductor material areselected such that the first region collimates at least part of thelight passing through the first region.
 6. The light emitting diode ofclaim 1, wherein the first region is column-shaped.
 7. The lightemitting diode of claim 1, wherein the first region has a cross-sectionthat is circular shaped.
 8. The light emitting diode of claim 1, whereinthe first region has a cross-section that is rectangular shaped.
 9. Thelight emitting diode of claim 1, wherein the first region is configuredto alter a light path of the light.
 10. The light emitting diode ofclaim 1, wherein the first region is configured to perform at least oneof focusing the light and increasing a light extraction efficiency ofthe light emitting diode.
 11. The light emitting diode of claim 1,wherein the second layer of semiconductor material comprises a secondregion partially enclosing the first region and including implantedions.
 12. The light emitting diode of claim 11, wherein a dose ofimplanted ions in the second region is at least the same as a dose ofimplanted ions in the first region.
 13. The light emitting diode ofclaim 11, wherein the first region and the second region are contiguous.14. The light emitting diode of claim 1, wherein the first region has arefractive index in the range of 2.1 to 2.55.
 15. The light emittingdiode of claim 1, wherein the light emitting diode is based on amulti-layer epitaxial thin film structure.